Fix exception in issue_filer (#592)
Fix exception in issue_filer (#592)

* Fix exception in issue_filer.

This fixes
```
argument of type 'NoneType' is not iterable (/base/data/home/apps/s~cluster-fuzz/cron-service:20190619t080320.419009350000290661/handlers/base_handler.py:211)
Traceback (most recent call last):
  File "/base/alloc/tmpfs/dynamic_runtimes/python27g/500e2bb923862643/python27/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 545, in dispatch
    return method(*args, **kwargs)
  File "/base/data/home/apps/s~cluster-fuzz/cron-service:20190619t080320.419009350000290661/libs/handler.py", line 95, in wrapper
    return func(self)
  File "/base/data/home/apps/s~cluster-fuzz/cron-service:20190619t080320.419009350000290661/handlers/cron/triage.py", line 289, in get
    issue_filer.file_issue(testcase, issue_tracker)
  File "/base/data/home/apps/s~cluster-fuzz/cron-service:20190619t080320.419009350000290661/libs/issue_management/issue_filer.py", line 322, in file_issue
    for result in apply_substitutions(label, testcase, security_severity):
  File "/base/data/home/apps/s~cluster-fuzz/cron-service:20190619t080320.419009350000290661/libs/issue_management/issue_filer.py", line 199, in apply_substitutions
    if marker in label:
TypeError: argument of type 'NoneType' is not iterable
```

google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz